n=int(input())
a=[[int(i) for i in input().split()] for i in range(n)]
b=[0 for i in range(n)]

for i in range(len(a)):
     b[i]=a[i][1]-a[i][0]

jud=0
for i in range(n):
    if b[i]!=b[0] or b[i]<0:
        jud+=1

if jud==0:
    print(b[0])
else:
    print(-1)